1. Field of the Invention
The present invention relates to a laminate type ceramic electronic component and a method for manufacturing the laminate type ceramic electronic component, and more particularly, relates to a laminate type ceramic electronic component including a multilayer ceramic substrate, a thick film resistor located on one principal surface of the multilayer ceramic substrate, and an overcoat layer located on the thick film resistor, and a method for manufacturing the laminate type ceramic electronic component.
2. Description of the Related Art
In the case of the formation of a resistor composed of a thick film on the surface of a ceramic substrate, the resistor is covered with overcoating of a glass based material for the purpose of protecting the resistor and improving the environmental resistance. While the resistor covered with the overcoating has a resistance value subjected to a fine adjustment with the use of an approach such as laser trimming, the resistor may have cracks caused after the laser trimming because an extremely strong thermal shock is applied to the resistor during the laser trimming. The resistor containing cracks cannot keep the resistance value stable.
The techniques which can solve this problem are described in, for example, Japanese Patent Application Laid-Open No. 8-250623 and Japanese Patent Application Laid-Open No. 2001-322831.
In the case of Japanese Patent Application Laid-Open No. 8-250623, in a method for manufacturing a ceramic substrate, which includes the respective steps of applying a resistor on the surface of a fired ceramic substrate, and further overcoat glass thereon, and subjecting the resistor and the overcoat glass to cofiring, the coefficient of thermal expansion of the resistor is set to be not less than the coefficient of thermal expansion of the overcoat glass so that the resistor has no cracks caused due to a thermal shock during laser trimming.
In the case of Japanese Patent Application Laid-Open No. 2001-322831, in a method for manufacturing a ceramic substrate substantially as in Japanese Patent Application Laid-Open No. 8-250623, the coefficient of thermal expansion of the overcoat glass is set to be lower than the coefficient of thermal expansion of the ceramic substrate so that the resistor has no cracks caused due to a thermal shock during laser trimming.
It is common that the resistor and the overcoat glass are formed by a post-fire process such as being sequentially applied on the surface of a fired ceramic substrate and subjected to firing, as described in Japanese Patent Application Laid-Open No. 8-250623 and Japanese Patent Application Laid-Open No. 2001-322831 described above. However, the adoption of the post-fire process increases the frequency of firing, and causes a problem of difficulty in the formation of fine patterns.
In order to achieve a reduction in the frequency of firing, the resistor and the overcoat glass may be subjected to cofiring with the ceramic substrate. However, in this case, when the behavior during the firing varies greatly between the ceramic substrate material and the overcoat material, it has to be considered that defects such as delamination and warpage of the substrate are more likely to be caused.
For the purpose of a reduction in the frequency of firing, the cofiring of the resistor and the overcoat glass with the ceramic substrate is described in, for example, Japanese Patent Application Laid-Open No. 2005-039164 and Japanese Patent Application Laid-Open No. 2005-174953.
Japanese Patent Application Laid-Open No. 2005-039164 discloses the formation of a conductor, a resistor, and overcoat glass on one principal surface of an unfired ceramic laminate with a plurality of glass ceramic green sheets stacked, further, the placement of a constrained layer containing, as its main constituent, a ceramic material which is not substantially sintered at a temperature at which the unfired ceramic laminate is sintered on the both principal surfaces of the ceramic laminate, and the firing of the unfired ceramic laminate with the conductor, the resistor, and the overcoat glass formed, as well as the constrained layers placed, which is carried out on the basis of a non-shrinkage cofiring process.
In addition, Japanese Patent Application Laid-Open No. 2005-039164 discloses, as preferred embodiments, that the glass component contained in the overcoat glass contains crystallized glass, the softening temperature of the glass component contained in the overcoat glass is higher than the softening point of the glass component contained in the resistor and the glass ceramic green sheets, and the glass component contained in the overcoat glass contains SiO2, Bi2O3, and B2O3.
However, in the technique described in Japanese Patent Application Laid-Open No. 2005-039164, when the preferred embodiments as described above are to be adopted, there is a need for the use of various types of glass that are different from each other between the substrate material and the overcoat material, and a difference is thus caused in shrinkage behavior in the non-shrinkage co-firing process, thereby causing the problem of difficulty in the integration of the substrate material and the overcoat material.
On the other hand, Japanese Patent Application Laid-Open No. 2005-174953 discloses that an unfired circuit board with a resistor and a surface layer conductor covered with ceramic green covering layers is sandwiched between constrained sheets and subjected to cofiring. In this case, the ceramic green covering layers mentioned above are constituted by a ceramic paste applied layer and a ceramic green sheet which are to be integrated by the cofiring into the unfired circuit board. In addition, the ceramic paste applied layers and ceramic green sheets constituting the ceramic green covering layers are supposed to have substantially the same composition as the ceramic green sheets constituting the unfired circuit board. In addition, among the ceramic green covering layers, the ceramic covering layer covering the surface layer conductor is removed along with the constrained sheets after firing in such a way that the surface layer conductor is thereby exposed at the surface of the substrate.
However, in the case of the technique described in Japanese Patent Application Laid-Open No. 2005-174953, the ceramic paste applied layers and ceramic green sheets constituting the ceramic green covering layers have substantially the same composition as the ceramic green sheets constituting the unfired circuit board, and there is thus no difference in coefficient of thermal expansion between the ceramic green covering layers and the unfired circuit board, thereby making cracks more likely to be caused after laser trimming, and possibly leading to a problem of difficulty in controlling the resistance value of the resistor stably. In particular, in the case of forming no overcoat layers on the ceramic green covering layers, this problem will be more significant.